2.1 Eye Structure
The eye, apart from its physical shape, has many of the structural features of the camera. The eyelids compromise the shutter. The iris is a diaphragm which contacts and dilates automatically in relation to the amount of light available. The eye has a lens composed of altered transparent epithelial cells but which is more elastic than the glass lens of a camera. The lens of the eye, as opposed to a camera, is suspended in such a way that muscle action can alter its shape and so change its focal length. As a consequence, the eye need not be shortened or elongated when objects at different distances are successively brought into focus as is necessary in a camera with a rigid lens. The a camera has its counterpart in the eye in strong connective tissue membrane, the sclera. The counterpart in the eye of the light-sensitive film used in a camera is a membrane of living cells of nervous origin, the retina, which lines not only the back but the sides of the eye as well. Then, finally, just as black is used to blacken all the interior surfaces of a camera that might leak or reflect light, black pigment is distributed generously between the retina and the sclera and in other sites where it would be useful.
However, unlike many modern cameras, the lens of the eye is not placed at its very front. The eye has its external aperture covered with a "glass window to keep out the dust", and its lens inside, a short distance behind the glass window. The transparent window in the central part of the front of the eye is of a curved form and is called the cornea. This is composed chiefly of a tough but transparent type of the dense connective tissue that is continuous with the opaque connective tissue of the sclera that surrounds and supports the remainder of the eye.
The cornea plays a larger role than the lens in focusing the image upon the retina because light retina because light rays are bent more in passing from air into and out of the lens.
The surface of the cornea is curved so that light rays coming from a single point source hit the cornea at different angles and are bent different amounts. But all in such a way that they are directed to a point after emerging from the lens.
The shape of the cornea and lens and the length of the eyeball determine the point where light rays reconverge. Although the cornea performs the greater part quantitatively of focusing the visual image on the retina, all adjustments of distance are made by changing the shape of the lens. Such changes are called accommodation. The shape of the lens is controlled by a muscle which flattens the lens when distant objects are to be focused upon the retina and allows it to assume a more spherical shape to provide additional bending of the light rays when rear objects are viewed.